The method disclosed herein relates to improvements in the well treating method disclosed in U.S. Pat. No. 4,250,965 issued Feb. 17, 1981 to Ben W. Wiseman, Jr., the disclosure of which is incorporated herein by reference in its entirety. The Wiseman patent discloses a method of stimulating primary production of a well wherein liquid carbon dioxide is injected down the bore of the well at a pressure sufficient for gaseous carbon dioxide, formed by the evaporation of the liquid carbon dioxide, to permeate through the strata and react with connate water to make carbonic acid. The well is then shut in for a period of time enabling the carbonic acid to react with the rocks in the strata to increase the permeability of the strata and thereby stimulate production from the oil well.
Corrosive acids such as hydrofluoric, hydrochloric, nitric, acetic, and carbonic acids are often used for stimulating the production from an oil or gas well. Conventionally, these aqueous acids are pumped at high pressure into the well to react with rocks in the oil or gas bearing strata in an attempt to increase the permeability of the strata. When the corrosive acids are being pumped down the production tubing string or casing, they react to some degree with the materials used to make the tubing or casing even when an inhibitor is mixed with the acid.
In some cases the formation may be damaged when the liquid is pumped into the formation at high pressure. Clay particles tend to swell and become disassociated from rock in the formation which may reduce permeability. The creation of an oil/water emulsion and the entrapment of water by surface tension also reduce permeability.
Conventional acidizing processes generally require the use of substantial quantities of water and complicated equipment to flush and swab the well to minimize damage to the tubulars in the well. Recovery and disposal of the water to prevent ecological damage is expensive and time consuming. Further, water introduced and left in the formation may damage the reservoir.
A paper entitled "The CO.sub.2 Huff 'N Puff Process" by Edmond L. Claridge presented at the Enhanced Recovery Week Symposium: EOR Using CO.sub.2 ", Dec. 6, 1984, at Houston, Tex., contains a review of several publications which discuss the injection of CO.sub.2 into an oil well alternating with production of the same well by a process referred to as "huff and puff." The article indicates that the cyclic CO.sub.2 injection process lowered viscosity of the mixture of CO.sub.2 and crude oil, swelled the crude oil in which CO.sub.2 was dissolved, provided a solution-gas drive achieved by dissolving CO.sub.2 in the crude oil up to a higher saturation pressure followed by lower pressures during production, and vaporization of components into CO.sub.2 with recovery of light crude oil components from produced CO.sub.2.
Liquid carbon dioxide and liquid hydrogen chloride have been used heretofore in an effort to increase the production of petroleum products from a well. U.S. Pat. No. 3,354,957 discloses a method for increasing the permeability of a subterranean formation comprising introducing substantially pure liquid anhydrous hydrogen chloride into the formation via a well bore to permit the hydrogen chloride to penetrate deeply into the formation and there form a strong effective acid with the irreducible water content of the formation. Before treatment, the well is pumped dry and filled with oil or another non-aqueous liquid. Liquid anhydrous chloride is then pumped down the tubing while the casing annulus valve at the well head is left open to permit discharge of the displaced oil at the surface. When a sufficient volume of the acid has been injected to displace the entire tubing string and the annular section opposite the pay zone, the annulus valve is closed and the pressure on the hydrogen chloride is then increased to force the hydrogen chloride into the formation where it vaporizes upon warming and upon contact with the connate water will ionize the yield hydrochloric acid which attacks and leaches the formation. Upon completion of the injection of the anhydrous hydrogen chloride into the formation, the disclosure states that the well is preferably shut-in and allowed to stand for from two hours to several days to permit the acid an opportunity to react. The residual hydrogen chloride, the hydrochloric acid which is formed and the gelatinous and insoluble salt materials are then removed from the formation and the well by flowing, swabbing or pumping, or during the course of ordinary production.
U.S. Pat. No. 3,411,583 discloses a petroleum recovery method in which a slug of fluid having a carbon dioxide content of at least 60 vol. %, desirably in a liquid state, a dense fluid or in gaseous form. Following injection of the fluid and subsequent injection of a sparingly oil-soluble gas such as nitrogen or air, the disclosure states that the well is preferably shut-in for a period of time to permit the carbon dioxide to become dissolved in the crude oil, and the sparingly oil-soluble gas to channel through and become distributed throughout the intended zone. The disclosure states that ordinarily a shut-in period of one to two days will suffice, and in general a period within this range is preferred, but a period of less than one day will suffice in some reservoirs while a period of more than three days will be required in others.
A paper entitled "CARBON DIOXIDE INJECTION INTO BOTTOM-WATER, UNDERSATURATED VISCOUS OIL RESERVOIRS", published in Journal of Petroleum Technology, pages 1248-1258 (October, 1977) describes an oil-recovery process that uses carbon dioxide in viscous, undersaturated oil reservoirs where the oil zone is underlain by bottom water. Seven mechanisms in a CO.sub.2 displacement process which are believed to contribute to improved recovery of oil are stated to include: (1) oil-viscosity reduction, (2) oil swelling, (3) energy supplied to the reservoir, (4) vaporization of crude oil, (5) blow down recovery, (6) stimulation effects, and (7) interfacial-tension effects. A field test is described in which liquid CO.sub.2 was pumped into the well and opened after a ten-day soak period. However, when the well was opened after ten days it produced 100% CO.sub.2 and the well head choke plugged with ice. The well was shut-in for another twelve days before back production began.
The authors concluded that cyclic injection-production of CO.sub.2 in bottom-water oil reservoirs does not appear to be a feasible recovery scheme. However, they concluded that a displacement scheme within injection of a slug of CO.sub.2 into the oil zone some distance from the producing well appeared to be an attractive recovery scheme for some bottom-water reservoirs.
An article entitled "CO.sub.2 -A NEW SHOT IN ARM FOR ACID, FRAC JOBS", published in The Oil and Gas Journal, Pages 92-97 (Oct. 8, 1962) describes a typical CO.sub.2 treatment in which a stream of liquid CO.sub.2 joined a stream of 5% retarded acid to form a mixed stream which was delivered into the well head. A paper entitled "CARBON DIOXIDE--A MULTIPURPOSE ADDITIVE FOR EFFECTIVE WELL STIMULATION" published in Journal of Petroleum Technology, Pages 237-242 discusses a well treatment process in which liquid carbon dioxide at 0.degree. F. and about 375 psi pressure was injected into a treating fluid just before it enters the well. The treating fluid flowed-back in 241/2 hours.
A review of references dealing with well stimulation processes in which gaseous acidizing agents are delivered into the formation reveal wide spread disagreement among persons skilled in the art of the effectiveness of the various well treatment methods. It is readily apparent that the disagreement of authorities regarding what the gaseouss material does and how it should be used has impeded progress.
U.S Pat. No. 3,405,761 discloses a process wherein a slug of steam at a temperature of about 500.degree. F. is injected into a well followed by a slug of nitrogen or air to displace water from the well before carbon dioxide is injected into the well. The carbon dioxide reacts with the steam condensate in the strata forming carbonic acid. After a soaking period of about one week, during which the carbonic acid reacts with the rocks in the strata, the injection well is opened to production.
Burnett U.S. Pat. No. 3,841,406 discloses a method of recovering oil from an oil-bearing formation in which a gas having a limited solubility in oil, e.g., carbon dioxide, is injected into the formation via a well, to increase the formation pressure, and thereafter a slug of carbon dioxide is injected via the same well. Following injection of the carbon dioxide, the well was shut in for a period of one to three days to permit the carbon dioxide to become dissolved in the oil to establish in the vicinity of the well bore a zone of oil saturated with carbon dioxide, so as to reduce the viscosity of the oil within a radial distance of about 20 to 100 feet from the well.
The technique of forming acids in situ with the oil or gas bearing strata in secondary and tertiary production wells in flooding projects are well known. Some examples of these techniques are described in U.S. Pat. Nos. 2,001,357; 3,072,185; 3,091,292; 3,252,512; 3,259,187; 3,344,858; 3,353,579; 3,392,782; 3,398,791; 3,405,761; 3,532,165; 3,929,192; and 4,250,965.
The method disclosed in the Wiseman '965 patent significantly stimulates production from the well. However, in certain well treating processes it appears that the use of an acid which is stronger than carbonic acid is needed to further increase the permeability of the strata without subjecting the production tubing string and casing to highly corrosive acid and attendant potentially ecologically damaging results of conventional acidizing processes.